SHEAR-STABLE VISCOSITY IMPROVER FOR LUBRICATING OILS

Abstract: Viscosity improver capable of imparting a VIE of at least 155 to a lubricating oil exhibiting, before incorporation of the said improver, a viscosity at 100 DEG C of 4.8 to 5.5 mm<2>/s and a VIE of 85 to 100, which exhibits a shear strength such that the relative drop in viscosity after four hours in the "VKA" test or after twenty hours in the "FZG" test is lower than 13 %.

Description: Title: SHEAR-STABLE VISCOSITY IMPROVER FOR LUBRICATING OILS FIELD OF THE INVENTION The invention relates to a shear-stable viscosity improver for lubricating oils, especially for mineral oils of paraffinic type. It is also aimed at the compositions based on this improver as well as at lubricating oils comprising an effective proportion of the said improver, optionally introduced by means of the said compositions. BACKGROUND OF THE INVENTION Viscosity improvers are usually macromolecular compounds. The objective of the invention is, above all, to provide a new viscosity improver for lubricating oils, which, besides the characteristics which are conventionally sought after, namely improving the viscosity index of the said oils, exhibits a markedly improved shear strength when compared with improvers that are already known. It is known that when lubricating oils are placed under extreme use conditions such as are encountered, for example, in gearboxes, differentials and gears of all kinds, especially in motor vehicles, the viscosity improvers incorporated in these oils are subjected to high shear stresses which cause them to lose their properties at least partially and which consequently lead to losses in viscosity in the lubricating oils. These losses in viscosity are of two types: they are, on the one hand, temporary losses in viscosity and, on the other hand, irreversible losses in viscosity. Temporary losses in viscosity are observed during operation; they arise from the aligning of the macromolecular chains which takes place in the shear field, and are reversible. Irreversible losses in viscosity are due to scissions of the macromolecular chains (D.E. Hillman, P.R. Marris, J.I. Paul and D. Pickles - Institute of Petroleum, vol. 008, 1977). Users of the oils of the kind in question are increasingly demanding and want the irreversible losses in viscosity, in other words the number of scissions of the macromolecular chains, to be as small as possible. To characterize the shear strength of a viscosity improver, a strength which is proportionately greater the lower the irreversible losses in viscosity of a lubricating oil treated with this additive, use is made of the following tests, according to which the said additive, which is predissolved in a formulating oil, is subjected to high shear stresses with the aid of a shearing tool. In general, use is made of two types of tools, the use of which then characterizes the test. In the first case this is a system of toothed wheels contained by a "FZG" machine and, in the other case, this is a bearing system with frustoconical needles contained in a "VKA" machine. In the first case, consequently, reference will be made to the "FZG" shear strength established by the standard CEC-L-37-T-85 and, in the second, to "VKA" shear strength determined according to standard VW 1437 or CEC PT-6. Procedure CEC-L-37-T-85 is a test procedure promulgated by the Coordinating European Council (CEC) for the Development of Performance Tests for Lubricants and Engine Fuels. This procedure is used to determine shear stability of polymer-containing oils employing the FZG (Forschungsstelle fϋr Zahnraderund Getriebebau) apparatus. Specifically, the purpose of the test is to determine the permanent viscosity loss of polymer- containing oils when a sample is mechanically stressed under the test conditions described. Special gear wheels are run in the lubricant under test in a dip lubrication system at constant speed for a fixed time. The bulk oil temperature is controlled and the loading of the gear teeth is set according to the procedure. At the end of the test period the oil is assessed for permanent viscosity loss and the difference in viscosity between the new and sheared oil is used to characterize the shear stability of the oil. The FZG gear test rig consists of drive and test gearing connected by two shafts. One shaft has a positive clutch for the application of the load. The test gear case contains a system for heating the test oil and a water-cooled coil to assist in cooling the test oil. A temperature sensor controls the heating system according to the preset temperature. The test rig is powered by a two speed electric motor at speeds of approximately 1500/3000 rpm. A sequence of operations is conducted wherein the test lubricant is subjected to the FZG test for 7.5 minutes each at load stages 1-4 followed by 19.5 hours at load stage 5, all at 90°C. The kinematic viscosity of the test oil is measured and the loss of viscosity is reported as percentage viscosity loss. The "VKA" procedure is also promulgated by the CEC. This procedure measures viscosity shear stability of polymer-containing transmission lubricants. This procedure provides a means for testing lubricating oils in a taper roller bearing to determine lubricant shear stability as characterized by a reduction in the kinematic viscosity of the lubricant. It allows conclusions to be drawn on the permanent viscosity loss to be expected under operating conditions, for example, in gear boxes, and which is caused by mechanical stress. The degree of shear stability is the relative viscosity loss (Rv) in % as defined by the following equation: Relative Viscosity loss Rv = [V r V f ] x 100% V: where: V* is the kinematic viscosity of the oil before shear in mm 2 /sec at 100°C V f is the kinematic viscosity of the oil after shear in mm 2 /sec at 100°C The smaller the value Rv the higher the viscosity stability. A lubricant is tested in a taper roller bearing fitted into a Four Ball EP Test Machine. The taper roller bearing runs submerged in 40 ml of lubricant at a constant speed and load with a test oil temperature of 60 °C during a defined number of motor revolutions (respectively during a defined running period). The kinematic viscosity of the test oil at 100°C is measured (in accordance with DIN 51562 or equivalent) before and after the test. The test conditions are given in the table below: Motor speed 1475 rpm + . 25 rpm Lubricant temperature 60°C ± 1 °C Test oil quantity 40 ml ± 0.5 ml Test load 5000 N + . 200 N Test duration A 348000 revolutions (approx. 4 hours) Test duration B 696000 revolutions (approx. 8 hours) Test duration C 1740000 revolutions (approx. 20 hours) N.B. : The basis for determining test duration or the number of revolutions of the motor was a theoretical speed for asynchronous motors of 1450 rpm. For motors with speeds that differ from the above, the test durations are to be corrected accordingly. Each test duration is conducted employing a fresh lubricant sample of known kinematic viscosity. After each test duration the kinematic viscosity at 100° C is determined for the sample and the relative viscosity change Rv is determined. The inventors have observed that the test performed with the aid of the FZG machine or "FZG" test induces at the end of twenty hours about the same irreversible drop in viscosity as the "VKA" test at the end of four hours; in both cases the viscosity changes from an initial value V* at a given temperature T to a final viscosity V f at the same temperature T; the stability to shear is then characterized by the relative drop in viscosity which is given by the formula shown below: V r V f x 100% V, More particularly, the objective of the invention is to provide a viscosity improver of the kind in question, which is not only capable of imparting a VIE of at least 155 to a lubricating oil exhibiting, before incorporation of the said improver, a viscosity at 100° C of 4.8 to 5.5 mm 2 /s and a VIE of 85 to 100, but which, in addition, exhibits a shear strength such that the relative drop in viscosity after four hours in the "VKA" test or after twenty hours in the "FZG" test is lower than 13 %, preferably lower than 12 % and, still more preferably, lower than 11 % . VIE is used to denote "Viscosity Index Extended", which is a quantity defined by ASTM standard D-2270 entitled Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 °C and 100°C, and which is represented by an arbitrary number employed to characterize the variation in the kinematic viscosity of a petroleum product with temperature. Additives of the kind in question which are already known, which consist of copolymers of esters of methacrylic acid with to C lg alkanols, exhibit a shear strength which, in the above conditions of the "VKA" and "FZG" tests, is reflected in a relative drop in viscosity of the order of 20%. SUMMARY OF THE INVENTION Accordingly, it is surprising that the properties of the viscosity improvers of the kind in question could be improved decisively and especially so that these additives satisfy the conditions referred to above, either by decreasing, to a value equal to zero in a copolymer obtained from esters of methacrylic acid with short alkanols and with long alkanols, the proportion of monomers based on short alkanols, the number-average molecular mass remaining constant, or, in the case of a copolymer of constant composition obtained from monomers consisting of esters of methacrylic acid with short alkanols and with long alkanols, the proportion by weight of short alkyl methacrylates being between 25 % and 75 % and preferably between 30 % and 50 %, by increasing the number-average molecular mass above a particular critical value which is characteristic for a given copolymer and starting from which the drop in viscosity in the VKA and FZG tests decreases. For purposes of this invention "short" refers to groups containing from 1 to 4 carbon atoms and "long" refers to groups containing about 10 to about 18 carbons. The abovementioned critical value of the number-average molecular mass is determined experimentally in each case. It follows that the viscosity improver in accordance with the invention is characterized in that it is capable of imparting a VIE of at least 155 to a lubricating oil exhibiting, before incorporation of the said improver, a viscosity at 100°C of 4.8 to 5.5 mm 2 /s and a VIE of 85 to 100, in that it has a shear strength such that the relative drop in viscosity after four hours in the "VKA" test or after twenty hours in the "FZG" test is lower than 13 %, preferably lower than 12 % and, still more preferably, lower than 11 %, and in that it consists either of a homo- or a copolymer essentially obtained by polymerization from at least one of the monomers of the group consisting of the esters of methacrylic acid with a long alkanol, more particularly C ι0 to C 18 and preferably C π to C 15 , the number-average molecular mass of this copolymer being from 7000 to 15,000 g/mol and, preferably, from 8500 to 13,500 g/mol, or of a copolymer essentially obtained by copolymerization from at least one of the monomers of the group consisting of the esters of methacrylic acid with a long alkanol, more particularly C 10 to C 18 and preferably C π to C 15 , and of at least one of the monomers of the group consisting of the esters of methacrylic acid with a short alkanol, more particularly C, to C 4 , the weight proportion of short alkyl methacrylates being between 25 % and 75 % and preferably between 30 % and 50 %, the number-average molecular mass of this copolymer being higher for a constant composition of the said copolymer than the critical value starting from which the drop in viscosity in the VKA and FZG tests decreases. According to an advantageous embodiment the abovementioned viscosity improver consists of a copolymer essentially obtained by copolymerization from 65 to 55 parts by weight of at least one of the monomers of the group consisting of the esters of methacrylic acid with a C n to C 15 long alkanol and from 35 to 45 parts by weight of at least one of the monomers of the group consisting of the esters of methacrylic acid with a to C 4 short alkanol, the critical value of the number-average molecular mass of this copolymer being from 27,000 to 32,000 g/mol. The copolymers constituting the viscosity improvers in accordance with the invention can be employed as they are, the quantity of copolymers used corresponding to a proportion of 2 to 40 %, preferably of 3 to 30 %, and, still more preferably, from 4 to 25 % by weight of the mass of lubricating oil to be treated. It is convenient, however, to use them in the form of a composition comprising the copolymers of this invention with a normally liquid organic diluent, preferably mineral oil, forming the reaction medium within which the copolymerization is performed. The mineral oil may be the same as the lubricating oil which is to be treated. The composition in accordance with the invention normally comprises from about 30 to about 90%, preferably from about 40 to about 80% by weight of at least one copolymer in accordance with the invention, the remainder to 100% consisting essentially of a normally liquid organic diluent, preferably mineral oil. The oils to be treated with the copolymers of this invention are oils of lubricating viscosity, including natural or synthetic lubricating oils and mixtures thereof. Natural oils include animal oils, vegetable oils, mineral oils, solvent or acid treated mineral oils, and oils derived from coal or shale. Synthetic lubricating oils include hydrocarbon oils, halo-substituted hydrocarbon oils, alkylene oxide polymers, esters of carboxylic acids and polyols, esters of poly carboxylic acids and alcohols, esters of phosphorus- containing acids, polymeric tetrahydrofurans, silicone-based oils and mixtures thereof. Specific examples of oils of lubricating viscosity are described in U.S. Patent No. 4,326,972 and European Patent Publication 107,282, both herein incorporated by reference for their disclosures relating to lubricating oils. A basic, brief description of lubricant base oils appears in an article by D.V. Brock, "Lubricant Base Oils", Lubrication Engineering, volume 43, pages 184-185, March, 1987. This article is herein incorporated by reference for its disclosures relating to lubricating oils. A description of oils of lubricating viscosity occurs in U.S. Patent No. 4,582,618 (Davis) (column 2, line 37 through column 3, line 63, inclusive), herein incorporated by reference for its disclosure to oils of lubricating viscosity. The lubricating oil composition in accordance with the invention is characterized in that it comprises at least one copolymer constituting the viscosity improver in accordance with the invention in a proportion of 2 to 40 %, preferably of 3 to 30 %, and, still more preferably, of 4 to 25 % by weight of the mass of lubricating oil to be treated. To prepare the copolymers constituting the viscosity improvers in accordance with the invention it is possible to make use of the conventional methods of radical copolymerization in solution in oil. Such methods are described in the work "Encyclopedia of Polymer Science and Engineering" (H.F. Mark, N.M. Bikales, C.G. Overberger and G. Menges), 2nd edition (1988), published by Wiley Interscience. These methods include free-radical initiated polymerization employing azo compounds or peroxides. Also described therein are photochemical and radiation initiated methods. Useful initiators include organic peroxides, hydroperoxides and azo compounds. Polymerization of acrylic and methacrylic monomers can take place under a variety of conditions, among which are bulk polymerization, solution polymerization, usually in an organic solvent, preferably mineral oil, emulsion polymerization, suspension polymerization and nonaqueous dispersion techniques. Solution polymerization is preferred, especially in mineral oil diluent. Molecular weights of the polymers can be controlled employing a number of techniques including choice of initiator, reaction temperature, concentration of monomers and initiator and solvent type. Chain transfer agents can be used. Molecular weights can be determined employing standard analytical methods such as gel permeation chromatography (GPC) using a polystyrene standard. Ionic polymerization techniques are known including cationic and anionic methods; however, cationic methods are generally ineffective for acrylate and methacrylate monomer polymerization. Free radical initiation is preferred. Because acrylic polymerizations are usually accompanied by liberation of considerable heat, care must be taken to avoid uncontrolled reaction. Temperatures can be controlled by using reactors with cooling jackets, controlling rates of addition and use of reaction solvents. A typical procedure for preparing the polymers of this invention is to charge at room-temperature about one third of the monomers, diluent, chain transfer agent and a portion of a peroxide initiator. The mixture is heated to about 90 °C at which time heating is discontinued and the temperature is allowed to rise exothermically, moderated with cold water cooling, if desired, to about 125 °C. At this temperature, the remaining two-thirds of monomer, additional oil, chain transfer agent and a portion of initiator are added over about 1.5 hours. During this time cold water cooling is applied, if desired, until the temperature drops to about 90 °C at which time any external cooling is discontinued. After monomer addition is completed the materials are held at 90 °C for 1 hour, then four additional portions of initiator are added at hourly intervals. After the final addition of initiator, the reaction mixture is held at 90 °C for 1 hour, stripped then diluted with oil to final concentration and filtered. Using these methods, a certain number of copolymers constituting the viscosity improvers in accordance with the invention have been prepared in oil solution by way of nonlimiting examples illustrating advantageous embodiments of the invention. EXAMPLE 1: Copolymers containing no short alkyl methacrylates Two copolymers constituting a viscosity improver in accordance with the invention are prepared by making use of the methods described above. To do this, two mixtures of esters of methacrylic acid with C π to C 15 alkanols are polymerized separately; in this case the percentages by weight of each of the esters based on the alkanols in the mixture of esters were approximately i Cβ Cβ / 4 / J = 1 % /20 % / 30 % /28 % /21 % by weight, in both cases. The polymerization conditions in the second case differ from those adopted in the first case in that the concentration of polymerization initiator in it is 1.9 times as high. The copolymers thus obtained were referred to as "Viscosity improver A" and "Viscosity improver B" . Their number-average molecular masses Mn are 13,000 g/mol and 8800 g/mol and their weight-average molecular masses 26,000 g/mol and 19,000 g/mol, respectively. Their polydispersity values P (ratio of Mw/Mn) are 2.0 and 2.2 respectively. EXAMPLE 2: Copolymers containing short alkyl methacrylates and long alkyl methacrylates A copolymer constituting a viscosity improver is prepared by proceeding as in Example 1. To do this, a mixture of 20 parts by weight of ester of methacrylic acid with methanol, of 20 parts by weight of ester of methacrylic acid with butanol and of 60 parts by weight of ester of methacrylic acid with a mixture of C n to C 15 alkanols is copolymerized, the percentages by weight of the alkanols in the alkanol mixture being Q 1 / Q 2 / C 13 / Cι 4 / C 15 = 1 % / 20 % / 30 % / 28 % / 21 %. The copolymer thus obtained was referred to as "Viscosity improver C" . Its number-average molecular mass Mn is 30,000 g/mol, its weight- average molecular mass Mw 76,000 g/mol and its polydispersity value P 2.5. In the case of this copolymer the critical molecular mass Mn determined experimentally is 28,000 g/mol. EXAMPLE 3: Comparison of properties Two additives according to the prior art are used for the purpose of comparison. The first of these additives ("Viscosity improver D") is obtained by copolymerization of a mixture of - 15 parts by weight of esters of methacrylic acid with methanol, - 85 parts by weight of ester of methacrylic acid with a mixture of C π -C 15 alkanols, the percentages by weight of the alkanols in this mixture being i / Qj / Cjj / Q 4 / Q 5 = 1 % / 20 % / 30 % / 28 % / 21 %. The number-average molecular mass Mn of this copolymer is 20,000 g/mol and its weight-average molecular mass 46,000 g/mol, the polydispersity value P being 2.3. The second of these additives ("Viscosity improver E") is also obtained by copolymerization of a mixture of - 15 parts by weight of ester of methacrylic acid with methanol, - 85 parts by weight of ester of methacrylic acid with a mixture of C n -Cι 5 alkanols, the percentages by weight of the alkanols in this mixture being i / 2 3 / C M / Q S = 1 % / 20 % / 30 % / 28 % / 21 % but the polymerization conditions differ from those adopted for the viscosity improver D in that the concentration of polymerization initiator is 1.5 times as high. The copolymer constituting the viscosity improver E exhibits a number- average molecular mass Mn of 12,000 g/mol, a weight-average molecular mass Mn of 23,000 g/mol and a polydispersity value P of 1.9. In contrast with the viscosity improvers in accordance with the invention, the two copolymers constituting the viscosity improvers D and E according to the prior art exhibit compositions of short alkyl methacrylates lower than 25 % but higher than 0 % . Compositions in accordance with the invention were prepared from the viscosity improvers A, B and C. Furthermore, compositions of the same type were prepared from the additives D and E. In the case of the additives A and B an oil of formulation K was employed (corresponding to the lubricating oil which was to be treated), the characteristics of which are: kinematic viscosity at 100°C: 4.8 mm 2 /s - VIE: 90. In the case of additive C an oil of formulation M was employed (corresponding to the lubricating oil which was to be treated), the characteristics of which are: kinematic viscosity at 100 °C: 5.3 mm 2 /s - VIE: 100. In the case of additives D and E, oil K was employed. The final additive concentration in these compositions was: A: 80 % by weight B: 80 % by weight C: 55 % by weight D: 69 % by weight E: 69 % by weight. The abovementioned compositions containing the additives A to E respectively were incorporated into the lubricating oils to be treated K and M and the following were then measured on the oils thus treated: - the kinematic viscosity at 100° C for A to E the kinematic viscosity at 40 °C for B and C the value of VIE and the VKA and FZG tests were performed. The values found are assembled in the table which follows, together with the final concentration of viscosity improver for each case in the lubricating oil treated. TABLE Viscosity Concentration Kinematic Kinematic VIE Drop in relative improver (%) in the oil viscosity at viscosity at viscosity (%) to be treated 100 °C 40 °C (mm 2 /-) (mm 2 /s) VKA FZG A 21.0 14.1 165 12.2 Invention B 28.4 13.9 89.1 160 4.8 C 7.9 9.1 47.7 175 9.6 D 20.0 15.0 175 20.1 Prior art E 20.2 14.1 170 18.1 When the results assembled in this table are examined, it is seen that the viscosity improvers of the prior art do not meet the conditions required in the case of the additives in accordance with the invention insofar as the shear strength is concerned.

Claims: What is claimed is: 1. A shear stable polymethacrylate viscosity improver for lubricating oils wherein each ester moiety of said polymethacrylate contains, independently, from about 10 to about 18 carbon atoms and the number average molecular mass ranges from about 7000 to about 15000 grams per mole. 2. The polymethacrylate of claim 1 wherein each ester moiety of said polymethacrylate contains, independently, from about 11 to about 15 carbon atoms. 3. The polymethacrylate of claim 1 wherein the number average molecular mass ranges from about 8500 to about 13500. 4. A shear stable polymethacrylate viscosity improver for lubricating oils derived from alkyl methacrylate monomers wherein from about 25 % to about 75 % by weight of said monomers are C alkyl methacrylates and the balance are C 10 . 18 alkyl methacrylates. 5. The polymethacrylate of claim 4 wherein the number average molecular mass is greater than the number average molecular mass of a corresponding polymethacrylate where the rate of change of drop in viscosity of a lubricating oil containing same which is subjected to shearing begins to decrease. 6. The polymethacrylate viscosity improver of claim 4 wherein from about 55% to about 65% by weight of said monomers are C n _ 15 alkyl methacrylates and from about 45% to about 35% by weight are C M alkyl methacrylates and the number average molecular mass ranges from about 27000 to about 32000 grams per mole. 7. Viscosity improver according to claim 4 characterized in that it consists of a copolymer obtained from 60 percent by weight of the esters of methacrylic acid with C π to C 15 long alkanols, 20 parts by weight of each of the esters of methacrylic acid with, on the one hand, methanol and, on the other hand, butanol, the number-average molecular mass of this copolymer being about 30,000 g/mol and its shear strength such that the relative drop in viscosity in the VKA test is 9.6. 8. Viscosity improver according to claim 1, characterized in that it consists of a copolymer obtained by polymerization from monomers of the group consisting of the esters of methacrylic acid with C π to C 15 long alkanols, the number-average molecular mass of this copolymer being 13,000 g/mol and its shear strength such that the relative drop in viscosity in the VKA test is 12.2. 9. Viscosity improver according to claim 1, characterized in that it consists of a copolymer obtained by polymerization from monomers of the group consisting of the esters of methacrylic acid with C n to C 15 long alkanols, the number-average molecular mass of this copolymer being 8,800 g/mol and its shear strength such that the relative drop in viscosity in the VKA test is 4.8. 10. A composition comprising from about 30 % to about 90 % by weight of a polymethacrylate according to claim 1 and the balance comprising a normally liquid organic diluent. 11. A composition comprising from about 30 % to about 90 % by weight of a polymethacrylate according to claim 4 and the balance comprising a normally liquid organic diluent. 12. A lubricating oil composition comprising an oil of lubricating viscosity and from about 2% to about 40% by weight of the polymethacrylate of claim 1. 13. A lubricating oil composition comprising an oil of lubricating viscosity and from about 2% to about 40% by weight of the polymethacrylate of claim 4. 14. The lubricating oil composition of claim 12 wherein the oil of lubricating viscosity is a mineral oil. 15. The lubricating oil composition of claim 14 wherein the mineral oil is a paraffinic oil. 16. The lubricating oil composition of claim 12 wherein the oil of lubricating viscosity has a viscosity at 100°C of 4.8 to 5.5 mm /sec and a viscosity index of about 85 to about 100. 17. The lubricating oil composition of claim 13 wherein the oil of lubricating viscosity is a mineral oil. 18. The lubricating oil composition of claim 17 wherein the mineral oil is a paraffinic oil. 19. The lubricating oil composition of claim 13 wherein the oil of lubricating viscosity has a viscosity at 100° C of 4.8 to 5.5 mm /sec and a viscosity index of from about 85 to about 100. 20. A method comprising the step of adding to an oil of lubricating viscosity from about 2% to about 40% by weight of the polymethacrylate of claim 1. 21. A method comprising the step of adding to an oil of lubricating viscosity from about 2% to about 40% by weight of the polymethacrylate of claim 4. 22. A polymethacrylate prepared by the process comprising reacting in an organic diluent in the presence of a free radical initiator at least one monomer selected from the group consisting of C 1 8 alkyl methacrylates. 23. A polymethacrylate prepared by the process comprising reacting in an organic diluent in the presence of a free radical initiator at least one monomer selected from the group consisting of C M alkyl methacrylates, and at least one other monomer selected from the group consisting of C 10 . 18 alkyl methacrylates.

Inventors: CANTIANI ROBERT, RICHARD JOEL, CANTIANI, ROBERT, RICHARD, JOEL
Applicants: CANTIANI ROBERT, LUBRIZOL CORP, RICHARD JOEL, CANTIANI, ROBERT, RICHARD, JOEL, THE LUBRIZOL CORPORATION
Application year: 1994
Main classification: C10M145
IPC codes: C08F20, C08F220, C10M145, C10N20, C10N30, C10N40